Experience with bilateral petrosal sinus sampling for the localization of ACTH secreting pituitary microadenomas has been extensively broadened. Over 50 patients have now been sampled and the results of surgery support the accurate localization provided by this technique. Patients with true microadenomas (less than 2 mm) have been cured by hemipituitary resection based on sampling results. In addition, thyrotropin secreting and prolactin secreting microadenomas have been successfully localized. Significance of Research: Our surgical results for pituitary microadenectomy in Cushing's disease are now better than other institutions by virtue of the fact that microadenomas, too small to be detected by the operating surgeon, can be confidently removed by basing a blind hemiresection on the basis of sampling data.